The present invention relates to a film end detector for use in cameras, and more particularly, to a film end detector disposed in a camera in which a film is wound by an electric motor.
Recently, an automatic film winding camera with a built-in electric motor has been widely used. In a conventional camera of the type, a film winding motor is automatically driven in response to the completion of a shutter operation and is automatically stopped in response to the completion of transport of a film length corresponding to one frame. Accordingly, the motor, even after an effective one frame of the film has been exposed, is driven when the shutter operation is completed to run idle without being able to further wind another one frame of the film, disadvantageously resulting in that a power may be dissipated or the film may be cut. Such disadvantages may be caused even when a film is accidentally caught within a camera.
To prevent such disadvantages, a conventional film winding camera with a motor is provided with a film end detector which detects a film end by a disabled condition of the motor. A conventional film end detector of the kind has a timer circuit in which a timing operation is initiated whenever the film winding motor starts its running and is reset whenever the one frame transport of a film is completed. Such a film end detector detects the film end when the timer circuit is not reset within a predetermined period.
However, the film end detector has a variety of disadvantages since the predetermined period is fixed. For example, when the film winding camera with a motor is further provided with a built-in electronic flash unit, the film transport performed while charging the flash unit requires a longer time period than without charging the flash unit so that the film end may be erroneously detected since the timer circuit is not reset even when in excess of the predetermined period. Because a dry cell, which is commonly used in the camera as a power source, supplies the motor with power while charging a main capacitor of the flash unit so that a supply current to the motor may decrease and the rotational torque thereof may be lowered.
Furthermore, the rotational torque of the motor also decreases when the dry cell is consumed so that the film transport may generally require a gradually increased period. This will result in an erroneous detection of the film end.
Still furthermore, a supply current from the dry cell to the motor will decrease at a low temperature, similarly causing an erroneous detection.
To prevent such erroneous detections, a method of previously setting a little longer period than the predetermined one may be considered. However, this method will be disadvantageous because of a delay of the detection of actual film end.